1. Field of the Invention
The invention relates generally to microwave heating of food items, and more particularly to a device for modifying the pattern of microwave radiation in a microwave oven cavity.
2. Description of Related Art
One of the problems associated with the use of microwave energy (i.e., electromagnetic radiation at a frequency of about 0.3 to 300 GHz) for cooking of food items in microwave ovens is nonuniformity of heating of the food items. Substantial temperature gradients may be present where uniformity of temperature is desired. The problem is in part due to the fact that power distribution in the oven cavity is typically non-uniform. The non-uniform power density gives rise to "hot spots" and "cold spots" within the cavity. Also contributing to the problem is the fact that a food item generally does not exhibit uniform temperature response to microwave radiation throughout its volume during a typical heating cycle. In particular, when a frozen food item is subjected to microwave radiation, certain portions of the item melt or thaw before other portions, and thawed portions absorb energy at a higher rate than frozen portions. Thus, the initial melting of a portion of the item results in a disproportionately high level of microwave energy absorption by the thawed portion, often resulting in severe overcooking of the thawed region, while other portions of the product remain frozen or undercooked.
Numerous efforts have been made to address the problem of uniformity of heating. Various types of susceptors, reflectors, and shields have been proposed as solutions. However, the utility of such devices has generally been limited. Devices which are useful in certain commercially available microwave ovens may not exhibit acceptable performance in other ovens, due to variations in power, cavity size and field configuration. Similarly, devices which enhance cooking of particular food items may provide little or no benefit in cooking other food items having different dielectric properties or different coefficients of thermal resistivity. Also, some devices which may improve cooking are unacceptable due to their tendency to cause arcing, sparking and/or combustion in the oven cavity. Furthermore, devices which shield a food item and/or reflect radiation away from the food item may increase the cooking time of the food item.
Typical food items have non-homogenous thermal resistivity and non-homogenous dielectric properties. Furthermore, the dielectric properties of such items often vary during cooking. These factors tend to further complicate efforts to improve cooking rate and uniformity.
One specific problem which has been addressed by various prior efforts is the cooking of frozen pizzas in microwave ovens. In the past, one approach to the problem has been to package the frozen pizza in a box which includes a susceptor film. The susceptor film may be dimensioned to correspond to the shape of the crust, and the crust of the pizza is placed on the susceptor film during microwave cooking. The susceptor film is intended to provide conductive heat transfer to the bottom of the crust, thereby browning and crisping the crust. However, it has been found that even with the use of susceptor films, uniformity of cooking of frozen pizzas remains difficult to obtain. The degree of difficulty varies somewhat from oven to oven, but typically, the center of the pizza remains at a lower temperature than the periphery of the pizza throughout the cooking cycle. Wide variations in temperature between points adjacent the periphery and points near the center of the pizza have been observed at the end of recommended cooking time for commercially available frozen pizzas.
U.S. Pat. No. 4,927,991 describes a grid for use in combination with a susceptor film. Example 13 of this patent describes use of the grip in combination with a French bread pizza product. The patent states that improved crispening of the crust was obtained, but that cooking time was increased by 15 seconds.
U.S. Pat. No. 4,266,108 discloses a microwave heating device in the form of a plate having a food product supported thereon, with shielding disposed over the food product, so that cooking of the food product is accomplished substantially totally by heat transferred to the food product from the plate.
U.S. Pat. No. 4,190,757 discloses a pizza package having a conductive metal foil shield bonded to the inside surface of the cover flap. The shield does not totally shield the food product contained in the package, but instead acts as a partial shield. Transmission is accomplished through openings of a predetermined size.
U.S. Pat. No. 4,904,836 discusses susceptors used to heat frozen pizzas in Example 7.
International Publication No. WO89/08373 discloses grids disposed above and below a food item such as a frozen hamburger patty, and also discloses at page 74 a conductive ring provided adjacent to and in close proximity with an outer edge of a food product. The application states that the conductive surface surrounding the food product preferably forms an electrically continuous loop around the food product. The application further states that a conductive ring may be used in combination with a grid or iris, and that the spacing of the conductive ring from the edges of the food product should preferably be about 0.75 in. or less.
U.S. Pat. No. 4,934,829 discloses a container for use in a microwave oven with a particular type of food product comprising a baked good topped with ice cream and a layer of sauce between the ice cream and the baked good. The top and sidewall of the container are shielded by foil stock, except for a lower portion of the sidewall which is unshielded to enable transmission of energy to the baked good.
It is a general object of the invention to provide improved methods and means of cooking frozen pizzas and other food products in microwave ovens. Further objects of the invention will become apparent below.